The color cathode ray tube used in television receivers and computer monitors provides excellent brightness, resolution and color display characteristics. While several variations of cathode ray tube structures have been developed through the years, most generally include a high strength funnel-like glass envelope having a faceplate which is either slightly convex or flat. The glass envelope further includes a rearwardly extending elongated neck within which a plurality of electron gun assemblies are supported. A pattern of phosphor deposits are formed on the inner surface of the faceplate and arranged in repeated patterns. Each phosphor type is characterized by the emission of a single color wavelength of light when struck by high energy electrons. In most cathode ray tubes, a selection electrode is interposed between the electron gun assembly and the phosphor screen of the faceplate. The selection electrode which typically comprises either a foraminous shadow mask or flat tension mask cooperates with the electron gun assembly in assuring that the electrons emanating from each electron gun toward the faceplate strike their respective phosphor deposits and avoid striking the phosphor deposits of each of the other two electron guns.
The cathode ray tube display is operated by imposing horizontal and vertical scan magnetic fields upon the electron beams causing the phosphor screen to be periodically scanned. The horizontal scanning system also produces the required high voltage electron beam accelerating potential which imparts sufficient energy to the electrons striking the phosphors of the faceplate to cause the desired light output. The control of the three electron guns to properly maintain and direct the electrons beams and their viewing screen impacts is a matter of considerable precision and is aided by supplemental magnetic fields applied to the electron beams for purposes of geometric correction and convergence correction.
Because the electron beam precision is extremely susceptible to the influence of magnetic fields, the metal components both within and proximate to the exterior of the cathode ray must be periodically degaussed to remove accumulated magnetism thereof. Such metal components are often subjected to unintentional or undesired magnetism due to the operation of appliances, tools and other sources of magnetic influence. The system is sufficiently delicate that the earth's magnetic field itself may cause undesired magnetism of sufficient strength to upset the CRT precision.
The high susceptibility of color cathode ray tubes to such magnetic influence has prompted practitioners in the art to employ various degaussing systems in the operation of color television receivers and color computer monitors. The most common degaussing system utilized an alternating current magnetic field of diminishing amplitude usually triggered for operation when the receiver or computer monitor is initially turned on. Perhaps the most common degaussing system provides one or more degaussing coils supported near the critical portions of the cathode ray which are energized by a diminishing amplitude alternating current usually derived from the receiver or monitor power supply. Most degaussing systems employ one or more thermally responsive devices such as thermistors to cause the required amplitude decay of degaussing current. While thermally responsive degaussing circuits are acceptable for many uses, they are subject to a cooling interval requirement between cycles. Thus, television receivers and computer monitors which are frequently turned on and off may receive little, if any, degaussing action during such use due to the lack of sufficient cooling intervals to permit the thermally responsive devices to cool down.
Accordingly, it is a general object of the present invention to provide an improved degaussing system. It is a more particular object of the present invention to provide an improved degaussing system which may be rapidly reset for successive operational cycles.